Probing Proton Spin Structure with Longitudinally Polarized p p Collisions at PHENIX

1
Probing Proton Spin Structure with Longitudinally
Polarized pp Collisions at PHENIX

• DIFFRACTION 2008
• La Londe-les-Maures
• September 13th, 2008
• Imran Younus

2
Proton Spin Crisis
EMC Experiment at CERN J. Ashman et al., Nucl.
Phus. B 328, 1 (1989)
Violates Ellis-Jaffe sum rule.
Proton-spin sum rule
?G measurement main RHIC-spin goal.

• Polarized DIS contribution of quarks to proton
  spin is amazingly small DS ? 0.1- 0.3
• Other candidate to carry proton spin Gluons
• Orbital angular momentum?

?? ?U ?D ?S ?G gluon polarization LZ
orbital angular momentum
3
Parton Distribution Functions (PDF)

• Quark distributions (q u,d,s)
• Unpolarized dist.
• Helicity dist.
• Transversity

• Gluon distributions
• Unpolarized dist.
• Polarized dist.

q(x,Q2)

g(x,Q2)
Dg(x,Q2)
Dq(x,Q2)
Universality! PDFs extracted from the data in one
process, can be used to make predictions for
other processes.
dq(x,Q2)

• PDFs - probability of scattering off of a parton
  carrying a particular fraction of the protons
  momentum.
• Polarized PDFs - the difference in probability
  between scattering off of a parton with one spin
  state vs. the other. Still as a function of the
  momentum fraction.

4
Pre-RHIC Era Situation
Asymmetry Analysis Collaboration M. Hirai, S.
Kumano and N. Saito, PRD (2004)

• Valence Dists - determined well.
• Sea Dist - poorly constrained.
• Gluon Dist can be either gt0, 0, lt0.

Gluon helicity distribution (?g(x)) remains
poorly constrained.
5
Along came Polarized pp Collider

• Strongly Interacting Probes
• Probes gluons directly
• Higher ?s ? clean pQCD interpretation
• Explores quark and anti-quark polarizations
  through W? production

DIS

• Polarized Gluon Distribution Measurements
  (?G(x))
• Use a variety of probes.
• Different probes different systematics.
• Use different energies ?s Access to different
  gluon momentum fraction x.

pp
6
Various Channels to Extract ?g(x)
7
RHIC as Polarized Proton Collider
Lmax 2 x 1032 s-1 cm-1 70
polarization 50 lt ?s lt 500 GeV

8
PHENIX Detector
High resolution at the cost of acceptance, High
rate capable DAQ, Excellent trigger capability
for rare events.

• Central Arms
• ? lt 0.35, ?? 2?900
• EM Calorimeter, photon trigger, ?, ?o, ?o
  detection.
• Ring Imaging Cherenkov Detector, e?
• Drift Chamber, charged hadrons.
• Muon Arms
• 1.2 lt ? lt 2.4
• Panels of Iarocci tubes/absorbers, Muon
  ID/trigger,
• Cathode Strip Chambers, Muon tracking, J/?, ??
• Global Detector
• Beam-Beam Counter (Quartz Cherenkov det.)
• 3.0 lt ? lt 3.9
• Zero Degree Calorimeter. ? gt 6.6
• Collision trigger, Collision vertex
  characterization,
• Relative luminosity, Local Polarimetry.

9
Beam Polarization
CNI Polarimeter Measures Left-Right Asymmetry in
elastic pC collisions. Provides fast relative
measurement of beam polarization.
Hydrogen Jet Polarimeter Measures Left-Right
Asymmetry in elastic pp collision. Provides
absolute measurement of beam polarization.
At PHENIX Interaction Point Bunch spin
configuration alternates every 106 ns. 111
Bunches max.
Transverse to Longitudinal at IP. Spin Pattern
Blue ? ? Yellow ? ? Provides all
possible combinations. Greatly reduces
possibilities of false asymmetries and
systematics due to time dependent det.
efficiencies.
10
Luminosity and Polarization
11
QCD Factorization
One can write cross-section as convolution of
PDFs and partonic sub process cross-section,
e.g., ? production,
Unpolarized case
Polarized case
Unpolarized, polarized parton distribution
functions, determined from experimental data.
Partonic level hard-scattering cross section.
Calculated by perturbative QCD. Process dependant.
Fragmentation function, determined from
experiment.
12
Unpolarized Cross-Sections in pp
Measured un-polarized cross-sections at ?s 200
GeV at RHIC are well described by NLO pQCD
calculations.
Necessary Confirmation that pQCD can be used
successfully at RHIC to extract PDFs.
pp ? ?o X PRD 76, 051106 (2007)
pp? ? X PRL 98, 012002 (2007)
13
Double Longitudinal Spin Asymmetries
ALL provides access to ?G.
At LO, pQCD give nonzero aLL for all sub
processes.
14
Measuring ALL
P1,P2 Polarization of the colliding beams. N
(N ?) experimental yields for same
(opposite) helicity collisions. R Relative
luminosity, determined at PHENIX using Minimum
Bias trigger counts.
R is measured using BBC and ZDC. Two independent
measurements are compared to determine
uncertainty on R.
Preliminary values for Run6.
15
Local Polarimetry At PHENIX

• Left-Right Asymmetry of Neutrons in transversely
  polarized pp was discovered at IP12.
• Use neutron asymmetry to estimate longitudinal
  and transverse components of polarization.
• ZDC/SMD make local polarimetry measurement at
  PHENIX.
• Allows us to measure transverse component of the
  longitudinally polarized beam.

For Run 5, the fraction of transverse component
is (PT/P)B 0.10 ? 0.02 (PT/P)Y 0.14 ?
0.02 ?PTB PTY ?/ ?PB PY ? lt (PT/P)B (PT/P)Y
0.014 ? 0.003 ?PB PY ? 0.24
16
ALL(?o) at ?s 200 GeV
PHENIX measures ?o??? decays using a highly
segmented (?? ? ?? ? 0.01 ? 0.01) electromagnetic
calorimeter (EMCal), at mid rapidity.
Fractional contribution to pp ??X at ?s200 GeV
at mid-rapidity
W. Vogelsang et al.
?o BG region 25 MeV around ?o peak BG region
Two 50 MeV regions Background contribution less
than 10 for pT gt 3.5 GeV. ALL(BG) consistent
with zero for both Run5 and Run6.
17
ALL(?o) Results From Run5 and Run6

• NLO pQCD
• pT (?o) 2?9 GeV/c ? xgluon 0.02?0.3
• This range represents 60 of the full integral of
  ?G(x) (GRSV).
• Each pT bin corresponds to a wide range in
  xgluon, heavily overlapping with other pT bins.
• The data is not very sensitive to variation of
  ?G(xgluon) within our x range.
• Any quantitative analysis should assume some
  ?G(xgluon) shape.

Run3,4,5 PRL 93, 202002 PRD 73, 091102 PRD 76,
051106 (2007)
GRSV models ?G 0 ?G(Q21GeV2)
0.1 ?G std ?G(Q21GeV2) 0.4 GRSV std
fits better to DIS data (as of ?2000).
Log10(xgluon)
18
?2 Test Based on GRSV
Calc. by W.Vogelsang and M.Stratmann

• std scenario, ?G(Q21GeV2) 0.4, is excluded
  by data on gt3 sigma level
• ?2(std) ? ?2min gt 9
• ?G G, ?G are rejected.
• The results are more consistent with ?G 0.
• Only exp. stat. uncertainties are included (the
  effect of syst. uncertainties is expected to be
  small in the final results).
• Theoretical uncertainties are not included.

19
Accessing Different xgluon Range

• The most likely xgluon for PHENIX ?o data in
  each pT point is xT /0.7, xT 2pT /?s
• Can access higher xgluon with higher xT.
• ?s 62.4 GeV gives access to higher xgluon.
• ?s 500 GeV gives access to lower xgluon.
• At fixed xT, the cross section is almost 2
  orders of magnitude higher at 62.4 GeV than at
  200 GeV.

xT 2pT /?s
20
Cross section and ALL(?o) at?s62.4GeV
?s62.4 GeV ?o cross section described well by
NLO/NLL pQCD calculations within theoretical
uncertainties.
Sensitivity of Run6 ?s62.4 GeV data collected in
one week is comparable to Run5 ?s200 GeV data
collected in two months, for the same xT 2pT/?s.
D. de Florian, W. Vogelsang, and F.
Wagner Phys.Rev.D76,094021(2007), arXiv0708.3060
21
Global Analysis
Global NLO QCD analysis of DIS, SIDIS, and RHIC
data in terms of the parton helicity
distributions.
Ref. D. de Florian et al. arXiv0804.0422v1
hep-ph
Includes PHENIX Run5 200 GeV, Run6 200 GeV
preliminary and Run6 62.4 GeV preliminary
results Also, STAR ALL(jets) data is
included. .. RHIC data set significant
constraints on the gluon helicity distribution,
providing evidence that ?g(x,Q2) is small in the
accessible range of momentum fraction.
22
ALL(?) at ?s 200 GeV

• No ? fragmentation functions (FFs) in the
  literature!
• Preliminary extraction of fragmentation
  functions using ee- data and pp??X data.
• Extraction uses method from DSS (deFlorian,
  Sassot, Stratmann, PRD75, 2007).
• Produced 1/2 as much as po , needs more
  statistics.

? has slightly enhanced sensitivity to qg (when
compared to ?o)
ALL(?) excludes GRSV min and max scenarios.
23
ALL(??-) at ?s 200 GeV

• Produced in large quantities.
• No real trigger at PHENIX.
• Charged pions above 4.7 GeV identified with
  RICH.
• At higher pT, qg interactions become dominant
  ?q ?g term.
• Different sensitivities of charged pions to ?u
  and ?d (combined with opposite signs of ?u and
  ?d) provide more sensitivity to sign of ?G
  through qg scattering.
• Needs more statistics.

W. Vogelsang et al.
24
ALL(?)

• pp ?? jet
• Theoretically cleanest mode for extracting ?G.
  No fragmentation function at LO.
• Gluon Compton Dominates. Sensitive to sign and
  magnitude of ?G.
• Small (?15) contamination from annihilation.
• NLO pQCD describes cross-section well ? can be
  used to interpret ALL(?).
• Rare probe, needs substantial statistics.

Fractional contribution of different channels to
the cross section.
25
ALL(?)
26
Jet kT Asymmetry
Net transverse momentum of a dilepton or dijet
pair

• intrinsic fermi motion of the confined quarks
  or gluons
• NLO radiation of an initial state or final
  state hard gluon
• soft Gaussian like distribution observed as
  pTpair ? 0 resummation.

• Another Possibility
• Spin-Correlated transverse momentum partonic
  orbital angular momentum.
• Assume other contributions have no spin
  dependence
• We can perhaps measure using jet kT spin
  asymmetry.
• Possible Effect in double longitudinal spin
• Idea proposed for the Drell-Yan process by
  M.Ta-chung et. al. (Phys. Rev. D40 p.769, 1989)
• Same idea for jets suggested by Douglas Fields.

27
Measuring jet kT di-hadron correlation
?o- h azimuthal correlation
Trigger on a particle, e.g. ?o, with transverse
momentum pTt. Measure azimuthal angular
distribution w.r.t. the azimuth of associated
(charged) particle with transverse momentum pTa.
The strong same and away side peaks in pp
collisions indicate di-jet origin from hard
scattering partons.
28
Measuring jet kT di-hadron correlation
Intra-jet pairs angular width ?near ? ?jT?
Inter-jet pairs angular width ?far ? ?jT?

• Zero kT

29
Measuring jet kT di-hadron correlation
Intra-jet pairs angular width ?near ? ?jT?
Inter-jet pairs angular width ?far ? ?jT?
? ?kT?

• Zero kT

• Non-Zero kT

30
Jet Kinematics
For details, see PRD 74, 072002 (2006)
31
Fitting the Correlation
32
kT Results for Unpolarized Case
??jT2? 585 6(stat) 15(syst) MeV ??kT2?
2.68 0.07(stat) 0.34 (syst) GeV
Phys. Rev. D 74, 072002 (2006)
33
Helicity Sorted kT

• Take the difference Like Unlike helicities
• Normalized by beam polarizations
• ??j2T? asymmetry (32 24 MeV out of 580 MeV for
  unpolarized)
• ??k2T?asymmetry (672 387 MeV out of 2.7 GeV
  for unpolarized)
• Final results soon to be published.
• Final uncertainty on kT asymmetry is reduced by
  almost a factor of 2, and the average is
  consistent with zero within one sigma.

34
ALL(quasi jets) at ?s200GeV

• ALL of jet-like clusters. Even with a limited
  acceptance in PHENIX central arm, we can capture
  most of a jet.
• Tag one high energy photon and sum energy of all
  nearby photons charge hadrons. No jet trigger
  at PHENIX.
• Not statistically separate from ?o and ?.

Definition of "pT cone Sum of pT measured by
EMCal Tracker with Real pT of jet is evaluated
by modified PYTHIA.
35
ALL(J/?) at ?s200GeV
Heavy quark production - Gluon Fusion dominates
at LO. PYTHIA estimate
Model dependant.
Rare probe, needs substantial statistics.
36
Summary

• RHIC is a unique facility, which provides high
  energy polarized proton-proton collsions.
• Allows to directly use strongly interacting
  probes (parton interactions) to study parton
  densities.
• High ?s make NLO pQCD applicable.
• Inclusive ?o data for ALL has reached high
  statistical significance to constrain ?G in
  limited xgluon range (?0.02 0.30).
• ?G is consistent with zero.
• Global Analysis of many channels together with
  DIS, SIDIS data will give us a more accurate
  picture of ?G(x).
• Need more statistics to explore different rare
  channels to study different gluon kinematics.
• Di-hadron correlations provide an analysis tool
  to measure jet kT in pp collisions.
• Measuring jet kT in longitudinally polarized
  collisions may provide access to spin-dependant
  coherent component of intrinsic kT , which one
  expects due to the orbital motion of partons
  inside the proton.
• Future planned and ongoing detector upgrades
  (silicon vertex detector, forward muon trigger
  upgrade, etc..) will provide access to even rarer
  probes.
• W measurements for flavor decomposition.
• DY measurements for possible access to orbital
  angular momentum.